The present invention is related to an improved polymerization method for preparing conductive polymer dispersions and an improved solid electrolytic capacitor formed thereby. More specifically, the present invention is related to the preparation of an improved conductive polymer dispersion with controllable particle size, specifically a particle size below about 100 nm, without additional post polymerization homogenization.
Solid electrolytic capacitors are widely used throughout the electronics industry. In high voltage applications, solid electrolytic capacitors with a solid electrolyte, formed by conductive polymer dispersions, give excellent high voltage performance compared to conductive polymer cathodes formed in-situ. These conductive polymer dispersions are prepared by a number of process steps including polymerization, purification, filtration, homogenization, evaporation, etc. Descriptions of these processes are provided in U.S. Pat. Nos. 5,300,575; 7,990,684; 7,270,871 and 6,000,840; U.S. Patent Publication No. 2011/0049433 and PCT Publication WO 2010/089111 each of which is incorporated herein by reference.
Capacitors and methods of making capacitors are provided in U.S. Pat. Nos. 7,990,683; 7,754,276 and 7,563,290 each of which is incorporated herein by reference.
The production process described in the prior art has the disadvantage of additional process steps, such as high-pressure homogenization, which are required to lower the particle size of the dispersion.
Polymerization of conductive polymer dispersion reactions are reported to be performed by various mixing methods such as magnetic stirring, stirring with agitators, high shear mixing and ultrasound irradiation. All of these mixing methods, except ultrasound irradiation, produce particle sizes in the range of 150-400 nm. A post polymerization homogenization is done after the polymerization to produce particle sizes below 150 nm.
Ultrasound irradiation is claimed to produce particle sizes below 100 nm directly during the polymerization. Although ultrasound induced polymerization has been used for many decades in synthetic polymer chemistry, this method of polymerization has not yet reached industrial maturity. Scaling up of ultrasound-induced polymerization is theoretically possible but issues, such as the efficient creation of cavitation bubbles, are difficult to achieve on an industrial scale. A further limitation to the application of ultrasound is the fact that several effects, such as radical formation and polymer scission or polymer degradation, may take place simultaneously which affects the long term performance of the polymer.
In all of these polymerization processes, there was neither a discussion nor a proposed method to control particle size during the polymerization method. Controlling the monomer droplet formation is very important to control the particle size. In various applications, such as a conductive polymers in solid electrolytic capacitors, there is a need for dispersions with various and controlled particle sizes. It would be advantageous if the polymerization could be controlled so as to control particle size or tune the particle size for a given application.
Thus, there is a need for a process for preparing conductive polymers with controllable particle sizes and with a lower particle size with less process steps without adversely affecting the polymer and device performance.
Commercially available intrinsically conductive polymer (ICP) dispersions are known. An exemplary ICP is polyethylene dioxythiophene:polystyrene sulfonic acid (PEDT: PSSA) dispersion, which is commercially available as Clevios P from Hereaus, which are stabilized by electrostatic or ionic stabilization with polycations stabilized with polyanion. Commercially available formulated ICP dispersions such as Clevios KV2, which is also commercially available from Hereaus and commonly used for solid electrolytic capacitors, are based on these types of ionically stabilized ICP dispersions.
There are several disadvantages for ionically stabilized dispersions including coagulation or gel particle formation of the higher solids dispersions over storage. In addition any changes in the electrolyte concentration could cause coagulation or gel particle formation. Due to the above disadvantages, some of the commercial formulated ICP dispersions have to be restabilized in some cases by more energy input into the system. The process of restabilization affects the performance as well as processing cost. Another aspect of the formulated ICP dispersions is that polymeric binders are added to the post polymerized dispersion to improve the coating performance for solid electrolytic capacitors. The presence of any unwanted electrolytes or ionics in these binders can affect the stability of the dispersion as well as affect the long term performance of the device. Electrostatic stabilization is known to have some disadvantages over other methods of stabilization as described in Polymeric Materials Encyclopedia, CRC Press, Vol. 1, Page 8025.